1. Field of the Invention
This invention relates to formable coatings utilizing phenoxy resin binders.
2. Description of the Prior Art
Thermoplastic poly(hydroxyethers) are well known in the art and are commonly referred to as phenoxy resins. Likewise, blends of such resins with relatively softer resins are known (see U.S. Pat. Nos. 3,925,504 and 4,337,330); blends of these types have been found useful to improve the flexibility of phenoxy resins in processing such as injection molding, or conversely to improve the processability of the resin into which the phenoxy is blended.
Phenoxy resins are also well known as coil coating resins, and in particular as binder resins in zinc-rich, corrosion-resistant coatings (see U.S. Pat. No. 4,391,855, commonly assigned U.S. Pat. No. 4,476,260, and commonly assigned patent, application Ser. No. 662,896, filed Oct. 19, 1984, now U.S. Pat. No. 4,529,756). Such coatings, while they provide excellent corrosion resistance on nondeformed surfaces, are often not fully satisfactory for uses in which the substrate and coating undergo substantial deformation, or where bake temperatures of less than about 400.degree. F. are used (so-called "low bake" systems). Typical of uses involving deformation are those involving the stamping and forming of sheet metal, as in the automobile manufacturing industry. As a result of such deformation, corrosion resistance is seriously degraded. While blending the phenoxy resin with softer resins has been reasonably successful for flexibilizing the phenoxy for, e.g., molding purposes, such blends have not proven to be satisfactory in the preparation of, e.g., solvent-borne, zinc-rich coating compositions. The problems encountered derive from a variety of factors: the need to maximize the concentration of the phenoxy resin in order to preserve its contribution to corrosion resistance, the need to achieve compatibility among the blended resins, the need for solubility in certain solvents preferred by the coatings industry, the need to achieve certain glass transition temperatures, and the like. Accordingly, it is desirable to flexibilize the phenoxy resin without significantly impairing either the corrosion resistance or the adhesion of the coating, while at the same time meeting the various processing needs indicated above. This unique balance of diverse properties has been realized by the instant invention.
In addition, with certain metal substrates it is necessary to use relatively low temperature bake cycles (peak metal temperature of less than about 400.degree. F.) in order to prevent the degradation of the good mechanical properties of the metal. With conventional phenoxy resins, it is not practical to use such low temperature bake cycles, because the material does not adequately flow and wet out the substrate to provide good adhesion, which is crucial to the performance of the coating. Using the blends of the instant invention it is possible to provide good adhesion and coating performance even with such low temperature bake cycles.
Further, there are a variety of applications, where the coating must be resistant to solvents or to hot water or steam and still remain highly formable. An example of such an application is in draw-redraw can coatings, where the coatings must undergo extensive elongation. Such coatings are typically crosslinked in order to enhance their blush resistance under steam sterilization conditions, but this usually leads to coating failure under deep-draw conditions. It has been found that the instant coating compositions contribute greatly improved resistance to failure and blush to such crosslinked, draw-redraw coatings applications.